Posts Tagged ‘nematodes’

After 38 years with the Cooperative Extension System at the University of Delaware I will be retiring officially at the end of June and this will be my last contribution to Weekly Crop Update. After three years as the assistant county ag agent in Kent County I had the opportunity to go to Newark and take the Extension Plant Pathologist position in the fall of 1977. It has been a wonderful career and I have had the pleasure of working with great colleagues at UD and our clientele in the agronomic crops, vegetable, and ornamentals industries. I will be out of the office beginning May 18. Plant samples will continue to be handled by Nancy Gregory in the Diagnostic Clinic as in the past. If you need help with any plant disease issues contact the county Extension agents first and they can help you, or find someone who can. As for the Nematode Assay Service, I have agreed to help continue this service in retirement. The Nematode Assay Service will continue to be part of the Plant Diagnostic Clinic, which will continue to be the point of contact for those services. There will not be any soil accepted for nematode testing from May 16 until July 1. If you have any questions please contact Nancy Gregory at 302-831-1390, or our website http://ag.udel.edu/plantclinic.

This must be a bad year for garlic because besides finding bulb mites we also have found garlic bloat nematode in several samples of damaged garlic. I wrote about this nematode last year and advised garlic growers to watch for it and to test any bad looking bulbs for it. This year growers are sending in their bad looking garlic bulbs and unfortunately many are infested with this nematode. The garlic bloat nematode Ditylenchus dipsaci can destroy a crop of garlic in one season. It probably came from Canada in garlic that was imported for food, but was planted as seed garlic. The problem then spread through distributors because there is no certification program for seed garlic and is it now widespread throughout New York. Symptoms of bloat nematode in garlic plants include: bloated, twisted, swollen leaves, distorted and cracked bulbs with dark rings (Fig.1).

Infested tissues become spongy, distorted and predisposes the plant to other problems like fusarium or white rot (Fig. 2) and bulb mites. These nematodes also can move to the inflorescence and remain in seeds for long periods of time in some plant species, i.e., beans, clover, and alfalfa where they are major sources of nematode dispersal. The nematodes can be spread around fields by equipment or on clothing and shoes. Garlic bloat nematodes can overwinter in soil or crop debris. If a grower has purchased or brought in new planting material over the last few years, especially if it came from Ontario or New York, you may have this pest. If you have not made any new introductions in a while you are probably safe. If you have garlic bulbs that look something like they do in figure 1 or 2 you should send a sample to a nematode laboratory for testing.

Fig. 1 The lack of roots on one side of plate and bulb deformation can be indicators of bloat nematode infection.

To prevent build-up of the nematode populations in a field, rotate away from any Allium crops (garlic, onions, and leeks) and control nightshades for at least 4 years. Another method to reduce levels of bloat nematodes in the soil is to keep the fields where garlic was grown moist, because bloat nematodes cannot survive for long periods in moist soils. They can persist for several years though, in dry soil and on dry plant residue. Bloat nematodes can actually survive better in dried crop debris than in soil.

Growers can use soil fumigants to reduce or eliminate the nematodes from infested areas of the field. Growers can also use bio-fumigant cover crops that can be planted after harvesting garlic. Mustard, sorghum-sudangrass have been shown to reduce nematode populations due to the bio-fumigant constituents they produce. Be sure to clean equipment and storage areas with meticulous sanitation techniques.

This is not a good time to take nematode samples for root-knot nematode detection but if soybean cyst nematode (SCN) is a concern and you are planting soybeans this spring it is not too late to sample. SCN is detectable all season long as long as the ground is not frozen or flooded. Remember that snap beans are also a host of SCN and fields planted to snap beans should be checked for SCN. Lima beans are resistant to SCN. Testing most of the major baby lima bean cultivars confirmed that they are resistant to the major race or genotype of SCN that is prevalent in Delaware.

Nematode sample bags are available at all the county Extension offices as well as the information sheet which needs to accompany each sample. This info is also available online at http://ag.udel.edu/extension/pdc/. Samples do not have to be submitted in these bags but are there for your convenience. Soil samples for nematode detection should be at least 2 cups of soil placed in a Ziploc plastic bag. Do not use paper bags unless they are double-bagged with a plastic bag.

Lima Bean Downy MildewDowny mildew is present now in Delaware lima bean fields. Levels are not high but many have been sprayed to protect the crop. Keep scouting and apply fungicides when needed; see past issues and the 2011 Delaware Commercial Production Recommendations for more information.

Fall Sanitation
In vegetable production it is not a good idea to leave old crop residue in the field any longer than necessary. If the crop is allowed to survive after harvest, fungi that cause many diseases continue to increase on the surviving plants. This allows higher numbers of the fungus to potentially survive until next season. Sanitation (plowing or disking the old crop) will help prevent pathogen carry-over.

Nematodes in Veggies
Fall is the best time to soil sample for nematode pests such as root knot, lesion, and other plant parasitic nematodes. After fall harvest but before any fall tillage is done take soil cores six inches deep between plants in the row. Samples should be taken in the root zone of the old crop. Twenty cores/ sample should be taken from random spots in the field and placed in a plastic bucket gently mixed, and a pint of soil submitted for analysis. Large fields should be subdivided into blocks of 10-15 acres each and sampled separately. Nematodes are not uniformly distributed in the soil and it would be easy to miss significant numbers if a single sample of 20- 25 soil cores represented a large acreage. Nematode test bags and instructions are available for purchase from the county Extension offices. Samples cost $10.00. Fall sampling for root knot nematodes is strongly recommended for fields that will be planted in cucumbers, watermelons, cantaloupes, lima beans or other high value vegetables where root knot could reduce production. Forms and instructions are also available on the web at http://ag.udel.edu/extension/pdc/index.htm.

New video on nematode sampling
“How to Sample for Nematodes” is a new video that was just produced to help growers with taking nematode samples in the fall to monitor plant parasitic nematode populations in their fields. The video features Bob Mulrooney, Extension Plant Pathologist at the University of Delaware explaining and demonstrating how to take soil sample for nematodes in row crops as well as narrow crop soybeans. The link for viewing is on the CANR You Tube server at http://youtu.be/x5HcY_L6aQk.

Fall is the best time to soil sample for nematode pests such as root knot, lesion, and other plant parasitic nematodes. After fall harvest but before any fall tillage is done take soil cores six inches deep between plants in the row. Samples should be taken in the root zone of the old crop. Twenty cores/ sample should be taken from random spots in the field and placed in a plastic bucket gently mixed, and a pint of soil submitted for analysis. Large fields should be subdivided into blocks of 15- 20 acres each and sampled separately. Nematodes are not uniformly distributed in the soil and it would be easy to miss significant numbers if a single sample of 20 soil cores represented a large acreage. Nematode test bags and instructions are available for purchase from the county Extension offices. Samples cost $10.00. Fall sampling for root knot nematodes is strongly recommended for fields that will be planted in cucumbers, watermelons, cantaloupes, lima beans or other high value vegetables where root knot could reduce production. Forms and instructions are also available on the web at http://ag.udel.edu/extension/pdc/index.htm.

Soybeans
Now is the time to start checking soybeans for soybean cyst nematode. Once soybeans have reached the 3rd trifoliate leave stage (roughly about 28-32 days from planting) the white or yellow female cysts can be seen on the roots. If you see irregular patches of stunted soybeans don’t presume the stunting is from drought. Digging the plants carefully may reveal SCN is present and could be the cause of the stunting. If you are seeing many cysts and stunting on resistant soybeans it is time to rotate out of that field to reduce SCN egg numbers.

White and yellow female soybean cyst nematodes on roots, 34 days after planting

CornThree corn samples arrived in the plant clinic this week with bacterial stalk rot. If you are irrigating from surface water sources, such as ponds or ditches, there is a risk of bacterial stalk rot. The bacterial can be in the irrigation water and get trapped in the whorl, the ear leaf sheath, and the ear shank. These places provide a place for water to sit and the bacteria can enter the stalks and cause a soft decay of leaf sheath, stalk, and ear shanks. It is foul smelling as well. It appears as random infected plants in the field and as a result it does not cause major losses. Corn is thought to be susceptible for a short period of time and the older the corn the less likely infection will occur. There is no chemical control for bacterial stalk rot. Treating irrigation water in the system with hypochlorite is an alternative solution.

Late BlightThere have been no new late blight detections in DE or VA on potatoes. The disease apparently is under control and the weather has not been very favorable, especially where the temperatures have been over 90°F. Besides the two finds in DE and VA the only active late blight at present appears to be on Long Island, NY on both potato and tomato.

Downy Mildew on CucurbitsAs most of you know by now downy mildew was found in Sussex County on Tuesday and Dorchester County, MD. Both finds were on pickling cucumber. Since then downy mildew was found in an additional field near Bridgeton, NJ, Talbot County, MD, Wyoming County, PA, and several more cucumber fields in NC. Now is the time to be spraying specific fungicides for downy mildew on cucumbers. Continue to check the IPM pipe website for more information on the spread of downy mildew: http://cdm.ipmpipe.org.

Root Knot NematodeRoot knot nematode can be a very yield limiting pathogen on very susceptible crops like cucumbers and other vine crops, lima beans, snap beans and tomatoes to name a few. They are often worse in very sandy soils or sandy knolls in fields. With the temperatures that we have seen here in DE you can begin to see the swellings or galls on the roots in about 21 days from seeding or transplanting. Plants in infested areas of the field will be stunted and if the plants are dug carefully, if root knot is present, you will see galls of varying sizes on the roots. We have no chemical controls except for vine crops once the nematodes are seen. Vydate should be applied preventatively in fields with known root knot infestations at seeding and/or later when plants are still small. See label for details. Treating early is always better than waiting until galls can be seen.

Root knot galls on baby lima bean roots, 23 days from planting

Pepper Anthracnose
Be on the lookout for anthracnose on peppers. It has been reported in southern NJ. Anthracnose fruit rot can be a very difficult disease to control if it gets established in a field. Fields should be scouted frequently especially if peppers or tomatoes have been planted in the past. It is best controlled by preventative fungicide sprays beginning at flowering. Apply Bravo or another chlorothalonil product every 7 days and alternate with a stroblilurin fungicide (FRAC code 11) like Cabrio or Quadris plus Bravo. If anthracnose fruit rot appears, removing infected fruit from heavily infected areas will help to reduce spore loads and reduce spread if done early and often enough. Fruit will need to be removed from the field and not just thrown on the ground.

There is a new ‘old’ pest infecting garlic and onions in New York and other New England states that has been found as far south as Pennsylvania. It is the garlic bloat nematode. The garlic bloat nematode Ditylenchus dipsaci is capable of severely damaging a field of garlic very quickly. It probably came from Canada in garlic that was imported for food, but was planted as seed garlic. The problem then spread through distributors because there is no certification program for seed garlic and it is now widespread throughout New York. Symptoms of bloat nematode in garlic plants include: bloated, twisted, swollen leaves, distorted and cracked bulbs with dark rings (fig.1). Infested tissues become spongy, distorted and predisposes the plant to other problems like fusarium or white rot (fig. 2). Garlic bloat nematodes can overwinter in soil or crop debris and can move to the inflorescence and remain in seeds for long periods of time in some plant species, i.e., beans, clover, and alfalfa, which act as major sources of nematode dispersal. The nematodes can be spread around fields by equipment or on clothing and shoes. If a grower has purchased or brought in new planting material over the last few years, especially if it came from Ontario or New York, you may have this pest. If you have not made any new introductions in a while you are probably safe. If you have garlic bulbs that look something like figure 1 or 2 you should send a sample to a nematode laboratory for testing.

To prevent build-up of the nematode populations in a field, rotate away from any Allium crops (garlic, onions, and leeks) and control nightshades for at least 4 years. Another method to reduce levels of bloat nematodes in the soil is to keep the fields where garlic was grown moist, because bloat nematodes cannot survive for long periods in moist soils. They can persist for several years though, in dry soil and on dry plant residue. Bloat nematodes can actually survive better in dried crop debris than in soil.

Growers can use soil fumigants to reduce or eliminate the nematodes from infested areas of the field. Growers can also use bio-fumigant cover crops that can be planted after harvesting garlic. Mustard, sorghum-sudangrass have been shown to reduce nematode populations due to the bio-fumigant constituents they produce. Be sure to clean equipment and storage areas with meticulous sanitation techniques.

Figure 1. The lack of roots on one side of plate and bulb deformation can be indicators of bloat nematode infection.

Root knot nematodes are microscopic roundworms that are widely distributed in Delaware agricultural soils and can cause varying degrees of damage to susceptible crops. Most of the damage caused by root knot nematodes is evident as stunting on vegetables such as pickling cucumbers, cantaloupe, watermelon, squash, and lima beans. Root knot populations are favored by the sandy, low organic matter soils in Kent and Sussex counties. There are several species of root knot nematodes that could occur here but Southern root knot nematode, Meloidogyne incognita is the most commonly found root knot species here. Most of the field crops grown in Delaware; corn, small grains, sorghum, alfalfa, Sudan grass, and Sudex are not good hosts of root knot and can reduce populations of root knot, but not eliminate them.

Small grains are a good rotation crop because if planting is delayed until soil temperatures at planting depth are below 65°F, root penetration does not occur. Soil temperatures are generally too low during most of the small grain growing season for root knot to increase. Small grains are poor hosts to begin with and the crop basically avoids infection or penetration by root knot. In my long career here I have never seen root knot nematode affect small grains (wheat, barley, oats, triticale, etc.).

Unfortunately soybeans, unless they are a resistant cultivar, are susceptible to root knot nematode. Soybeans can tolerate low populations of root knot without producing visible symptoms but yield loss can occur depending on growing conditions, especially low rainfall. High populations and adverse growing conditions can cause stunting as severe as that produced by soybean cyst nematode. Root knot resistance has been available in soybean for a long time but it has been in group 5 or later maturity groups. Recently, advances have been made to incorporate resistance into group 4 varieties. There are a few resistant group 4s available that were posted on the VIPS website (http://www.vipsoybeans.org): Schillinger 479.RC, Southern States RT 4470N, HS HiSoy FS 41T80, FS HiSoy HS 4426, FS HiSoy HS 46T80. There should be an updated list on the VIPS site soon of soybean varieties rated in 2010. Another source of information is a soybean variety selector from North Carolina. There are no group 4, only group 5 resistant soybeans on this site http://www.soybean.ncsu.edu/soyvar/. Root knot resistant soybeans would be an excellent rotation crop for vegetable growers who plant susceptible fresh market or processing vegetables.

Field corn varies in its susceptibility to southern root knot nematodes. There is no current data on corn hybrid screening for resistance to root knot nematodes. Earlier studies indicated that there was wide variation in susceptibility to root knot in corn. A test conducted at Auburn University in 2009 of hybrids grown in the South showed that none were resistant. Irrigated corn is not likely to be damaged by low to medium root knot populations, but can support damaging population levels if followed by a susceptible vegetable crop. It might be very difficult to even see symptoms of high root knot populations in field corn especially if it is irrigated. Nematode numbers in corn seem to be increasing but it is difficult to know for sure. It has been thought that the increase in Bt corn and the shift to pyrethroid insecticides has had an impact. Growers are not using the granular and liquid carbamate and organophosphate insecticides at planting which would suppress nematode populations as well as control the target insects.

Managing Root Knot Nematodes
Rotation is often a limited control strategy for root knot because it has such a wide host range. Alfalfa and oats are thought to be the safest crops to use in a rotation to reduce root knot nematodes. Increasing organic matter in fields with low organic matter and high levels of root knot or other plant parasitic nematodes can have a suppressing effect on root knot populations. Fall planted rape and other mustards may also be useful to suppress root knot populations as a biofumigant when they are plowed under before they go to seed in the spring prior to planting the crop. Rape can be infected with root knot if populations are high and soil temperatures are above 65°F at planting or the fall is warm. (See under small grains above). It is the decomposition of the plant parts when tilled into the soil that releases the chemicals that kill the nematodes, not root exudates from living plants. Soil sampling in the fall right after harvest is the best way to know if you have high root knot populations in your soil. Unfortunately spring is not the best time to sample because the nematode overwinters primarily as eggs which are not detected in the methods used for processing soil samples for nematode analysis. Soil sampling this time of year can underestimate the number present or not detect low populations.

Symptoms

Typical stunting from root knot nematode in an irregular area of a lima bean field

Heavily galled and stunted lima bean roots dug from an infected area. Soybeans can be this badly infected as well. Corn and other field crops produce much smaller and less evident galling

There is a new sample submission form available on the Plant Clinic website http://ag.udel.edu/plantclinic. This new form is updated to reflect personnel changes and lines for entering email addresses and information that will help us respond as quickly as possible. Please fill out a sample submission form each time you submit samples. These are also available from the county Extension offices when you purchase the soil bags for nematode analysis.